If you'd like to buy 18+ rated games on the Wii U's European eShop, you'll have to…
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As it turns out, the policy exists in order to comply with German law, which requires that any content rated 18+ (the European equivalent to America's "M" rating) be sold at night.

Nintendo has confirmed this in a statement sent to Eurogamer. From that statement:

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"Legal age restriction requirements vary across a number of European countries. Since Nintendo of Europe is based in Germany, Nintendo eShop is complying with German youth protection regulation which therefore applies to all our European markets. Under German law, content rated 18+ must be made available only at night.